1. Field of the Invention
The present invention relates to high speed imaging using time delay integration (TDI) sensors and in particular to an analog front end (AFE) and an analog to digital converter (ADC) that can be used in conjunction with the TDI sensors to form a high density digitizer (HDD).
2. Related Art
Time delay integration (TDI) is an image scanning process that produces a continuous image of a moving two-dimensional object. In a TDI system, image photons are converted to photocharges in a two-dimensional array of pixels. As the object is moved, the photocharges are shifted from pixel to pixel down the sensor, parallel to the axis of movement. By synchronizing the photocharge shift rate with the velocity of the object, the TDI can integrate signal intensity at a fixed position on the moving object to generate the image. The total integration time can be regulated by changing the speed of the image motion and providing more/less pixels in the direction of the movement. TDI inspection systems can be used for inspecting wafers and/or reticles.
Conventional inspection systems configure TDI sensors to drive pre-amplifiers and analog-to-digital converter (ADC) devices through long traces. These long traces can introduce noise and loading, both of which can undesirably reduce system performance. The signal level is also obtained in digital domain by subtracting two consecutive samples. This subtraction results in the ADC operating at twice the sampling rate and also more thermal and quantization noise, which results in a lower signal-to-noise ratio (SNR). Moreover, the high channel count (resulting from reading out sets of pixels in parallel) and high-density requirements associated with high-speed TDI sensors generally increase board complexity and expense.
Therefore, a need arises for a compact multi-channel analog front end and digitizer for high speed imaging applications.